Implement ??= operator
??= is a lazy, conditional assignment. Whereas a ?= immediately assigns to
the variable if the variable has not yet been set, ??= does not apply the
default assignment until the end of the parse. As a result, the final ??= for
a given variable is used, as opposed to the first as in ?=.
Note that the initial implementation relies upon finalise() to apply the
defaults, so a "bitbake -e" without specifying a recipe will not show the
defaults as set by ??=. Moving application of the default into getVar adds
too large a performance hit. We may want to revisit this later.
(Bitbake rev:
74f50fbca194c9c72bd2a540f4b9de458cb08e2d)
Signed-off-by: Chris Larson <chris_larson@mentor.com>
Signed-off-by: Richard Purdie <rpurdie@linux.intel.com>
